About append-transform.c

This example is based on null-transform.c. It appends text to the body
of an HTML text response document on its way from the cache to the
client.   

The plugin first makes sure that it has received a "200 OK"
response from the origin server.  It then verifies that the returned
document is of type "text/html".  It "transforms" the document by
appending text to the file.   To read and write to the body of the 
document, the plugin uses functions similar to those in null-transform.c.

You place the text to be appended in a file, and you must provide the 
path to the file in plugin.config. In other words, plugin.config must 
have a line similar to the following:

On NT: AppendTransform.dll path/to/file
On Solaris: append-transform.so path/to/file

Specify an absolute path to the text file, or a relative path as
described in the file-plugin README.

INKPluginInit does the following:

- makes sure that there is a text file specified in plugin.config
	(if not, it returns an error message)

- calls the function load to load the contents of the file into
	a buffer to be appended to HTML response bodies

- sets up the global hook to call back the plugin:
	INKHttpHookAdd(INK_HTTP_READ_RESPONSE_HDR_HOOK, 
INKContCreate (transform_plugin, NULL)); 

The load function does the following (similar to file-1.c):

- opens the file specified in plugin.config using
	INKfopen

- creates a buffer for the text to be appended using
	INKIOBufferCreate

- creates a buffer reader for the text to be appended using
	INKIOBufferReaderAlloc

- reads the contents of the file in to the buffer using:
	INKIOBufferStart
	INKIOBufferBlockWriteStart
	INKfread	
	INKIOBufferProduce
	INKIOBufferReaderAvail

- closes the file using
	INKfclose

The transform_plugin function does the following:

- tests the response body to make sure it is text/html, using 
	the function "transformable". The transformable function
	uses the following API calls:
	INKHttpTxnServerRespGet
	INKHttpHdrStatusGet	
	INKMimeHdrFieldFind
	INKMimeFieldValueGet

- if the response body is deemed transformable, transform_plugin calls
	transform_add 

- continues the HTTP transaction using
	INKHttpTxnReenable

The transform_add function does the following:

- creates a continuation for the append transform, using
	INKTransformCreate (append_transform, txnp); 
	The handler function for this continuation is 
	append_transform.

- adds a transaction hook for the append transform, using
	INKHttpTxnHookAdd(txnp, INK_HTTP_RESPONSE_TRANSFORM_HOOK, connp);
	This transaction hook sets up a callback during transactions. 
	When the event INK_HTTP_RESPONSE_TRANSFORM_HOOK happens,
	the append_transform function is called back.

The remaining functions in the plugin, append_transform and 
handle_transform, are similar to null_transform and 
handle_transform in the null-transform.c plugin.